io_uring: remove duplicate io_uring_alloc_task_context() definition

This function exists in both tctx.h (where it belongs) and in io_uring.h
as a remnant of before the tctx handling code got split out. Remove the
io_uring.h definition and ensure that sqpoll.c includes the tctx.h
header to get the definition.

Signed-off-by: Jens Axboe <axboe@kernel.dk>